The development of artificial photosynthesis technology that replicates photosynthesis of plants to electrochemically convert sunlight to a chemical substance has been recently progressing in consideration of an energy problem and an environmental problem. Converting sunlight to a chemical substance to store it in a cylinder or a tank is advantageous in that it costs lower for energy storage and has a less storage loss than converting sunlight to electricity to store it in a battery.
A known electrochemical reaction device that electrochemically converts sunlight to a chemical substance is, for example, a two-electrode device including an electrode having a reduction catalyst for reducing carbon dioxide (CO2) and an electrode having an oxidation catalyst for oxidizing water (H2O), the electrodes being immersed in water containing dissolved carbon dioxide. In this case, these electrodes are connected to each other via an electric wire or the like. The electrode having the oxidation catalyst oxidizes H2O using light energy to produce oxygen (1/2O2) and obtain a potential. The electrode having the reduction catalyst obtains the potential from the electrode that causes the oxidation reaction, thereby reducing carbon dioxide to produce formic acid (HCOOH) or the like. In the two-electrode type device, since the reduction potential of CO2 is obtained by two-stage excitation, conversion efficiency from the sunlight to chemical energy is very low, namely, about 0.04%. Another known electrochemical reaction device that performs a photoelectric conversion using GaN, oxidizes water at a surface thereof and reduces CO2 by a copper plate which is connected thereto. However, conversion efficiency is low, namely, 0.2%.
An electrochemical reaction device using a stack (silicon solar cell or the like) where a photovoltaic body is sandwiched between a pair of electrodes is also under consideration. In the electrode on a light irradiation side, water (2H2O) is oxidized by light energy, whereby oxygen (O2) and hydrogen ions (4H+) are obtained. In the opposite electrode, by using the hydrogen ions (4H+) produced in the electrode on the light irradiation side and potential (e) generated in the photovoltaic body, hydrogen (2H2) or the like is obtained as the chemical substance. There is also known an electrochemical reaction device where the silicon solar cells are stacked. The conversion efficiency from the sunlight to the chemical energy in this case is high, but it is inconvenient to store and transport hydrogen. In consideration of energy problems and environmental problems, it is preferable to convert into not hydrogen but carbon compounds which are easy to be stored and transported.